FIG. 1 schematically illustrates a typical turbofan engine 10 for subsonic flight. The engine 10 generally comprises in serial flow communication a fan 12 through which ambient air is propelled, a multi-stage compressor 14 for pressurising the air, a combustor 16 in which the compressed air is mixed with fuel and ignited for generating a stream of hot combustion gases, and a turbine section 18 for extracting energy from the combustion gases. The engine 10 also comprises an auxiliary or accessory gearbox (AGB) 20 on which are located mechanical and electrical systems, such as fuel pumps, oil pumps, generators and a starter/generator. The main rotating parts of the engine 10 are connected in two subgroups, the low pressure (LP) spool and the high pressure (HP) spool, which are coaxially disposed. In use, the engine 10 is started by the starter which is mechanically connected to the HP spool using a set of gears and a tower shaft 22. Once the desired HP spool speed is reached, fuel is provided into the combustor 16 and is ignited to start or “light” the engine 10.
When the engine 10 is mounted on an airplane, in the unlikely event of a flame out or engine shutdown, dynamic pressure due to forward speed of the airplane creates a windmill effect to spin the LP and HP spools. This spinning is then further assisted by the starter to spin the HP spool up to the starting speed so that relight can successfully occur. In other arrangements, a shaft power transfer arrangement is provided to transfer windmilling energy from the LP spool to the HP spool to assist acceleration of the HP spool to relight speed. However, there is a continuing need for simpler and better systems.